In a microwave radio communications system for example using QAM (quadrature amplitude modulation), it is well known to mix an IF (intermediate frequency) signal with an LO (local oscillator) signal at a microwave carrier frequency to produce an RF signal which is amplified in a power amplifier and transmitted. As is also well known, such a power amplifier generally has a transfer characteristic which is increasingly non-linear at increasing power levels, eventually reaching a clipping level at which the output amplitude no longer increases with increasing input amplitude, i.e. the output is clipped. It is known to predistort the input to the power amplifier to compensate for its non-linearity, and to limit or back off the input signal amplitude to ensure that the clipping level of the power amplifier is not reached.
A problem with the latter technique is that the amount of back-off which is necessary is difficult to determine and is difficult to realize with accuracy. Due to the limited bandwidth of the radio communications channel, there is a signal overshoot, dependent upon the probability distribution function of the signal, which must be accommodated by the power amplifier. In other words, the peak power of the signal must be reduced by a certain amount, referred to as back-off, below the clipping level of the power amplifier. The required back-off is also affected by particular characteristics of the power amplifier, such as its temperature and output transistor characteristics. In addition, monitoring of the actual output power of the amplifier can be inaccurate due to tolerances of the power monitor and its coupling to the amplifier output.
For these reasons, and the fact that clipping due to an insufficient back-off results in transmission errors, known microwave radio communications systems tend to use a back-off which errs on the side of being too large rather than too small. For example, a 64-QAM system, for which clipping by 2 dB produces symbol errors, which has a 10 watt saturated power amplifier may typically be operated with a 10 dB back-off to provide a maximum output power of only 1 watt.
With microwave radio communications systems increasingly using more QAM states and requiring increased dynamic range, there is an increasing need to improve signal-to-noise ratio, and hence the maximum output power of the power amplifier. Accordingly, it is desirable to reduce the back-off to the least possible amount. However, with such systems using more QAM states the production of errors due to any clipping which does occur is exacerbated. For example, with a 512-QAM system using a 36 MHz symbol rate in a 40 MHz channel, clipping of signal overshoots by only 0.5 dB produces symbol errors.
A transmitter in a microwave radio communications system can be operated with either a fixed or a variable output power. With a fixed output power, the transmitter is continuously operated at a full output power with a back-off which is sufficient to ensure a transmit error rate equal to or better than the residual symbol error rate which is desired in unfaded operation of the system. For a variable output power, a reverse signalling control channel is provided from the receiver back to the transmitter to control the transmitter output power. In this case full transmitter output power is only used when necessary due to fading, or transmission path loss, conditions. The transmit error rate at full power is then only required to be better than the faded receive error rate, which is several orders of magnitude greater than the unfaded residual symbol error rate referred to above. Consequently, variable output power operation requires less back-off at full power operation of the transmitter.
However, even with variable output power operation it is still necessary to provide a back-off which is sufficient to minimize errors but is desirably no greater than this.
Accordingly, an object of this invention is to provide a method of controlling clipping in a microwave power amplifier in such a manner that the back-off is just sufficient to provide a desirably low error rate.